The present invention relates to the field of the maintenance of the screens used in printing, and more particularly in screen printing.
In screen printing, screens are used which consist of a frame in which a fabric is stretched, originally made of silk but nowadays more usually made of nylon/polyester, which has very fine mesh cells (of the order of 100 xcexcm). A masking product, intended to fill some of the mesh cells of the fabric, is firstly applied to the screens and then ink is applied, which passes through the mesh cells which are left free and which correspond to the impression of the pattern to be reproduced on the printing medium.
For each new pattern to be printed using this technique, it is necessary to clean the screen, removing the ink and the masking product.
Conventionally, the screen is cleaned in three phases, which are as follows:
in a first phase, the screen is treated with an organic solvent which allows the ink to be removed and then it is rinsed with high-pressure water in order to remove the solvent and the ink residues;
in a second phase, an aqueous solution of an oxidizing agent, such as sodium periodate, called a degritting solution, is applied in order to make the masking product disappear from the obstructed mesh cells, and then a further rinsing with high-pressure water is carried out; and
in a third phase, the ghost image remaining on the screen is removed by means of a chlorinated product and a final rinse with high-pressure water is carried out.
This cleaning is carried out either manually, or automatically in washing tunnels in which the steps take place in succession in different compartments.
The machines currently used for this cleaning have four compartments. Associated with each compartment is a tank designed to contain a product corresponding to the aforementioned phases.
In the first compartment there is a tank containing a solvent able to dissolve the inks used in screen printing. In this compartment, the screen is sprayed with the product found in the tank by nozzles which are mounted on a vertical spray bar. The vertical spray bar moves in a horizontal direction in order to spray the entire area of the screen. The sprayed product falls back into the tank, by gravity, with the ink.
The second compartment also has a tank containing solvent. The latter is also sprayed using a vertical spray bar which moves in a horizontal direction over the screen. This solvent is cleaner than that of the first tank. Solvent-recycling machines in the first and second tanks are provided in order to increase the lifetime of the product contained in these tanks.
Before the screen passes from the second to the third compartment, it is washed with high-pressure water.
The corresponding tank in the third compartment contains a degritting product allowing the masking product to be removed. This product is applied to the screen, also using nozzles placed on a vertical spray bar which moves horizontally.
In the final, fourth compartment, the screen is washed with high-pressure water.
The screen is then ready to undergo a new step, that of removing the ghost image.
While the screen is being cleaned, it is therefore out of commission for a period which may range up to one day, or even longer.
This is because, and by way of example to illustrate the loss of time devoted to cleaning a screen, the final phase consisting in removing the ghost image is carried out by applying, to both sides of the screen, a product based on chlorine and sodium hydroxide, which is left to act for at least half a day. After this reaction time, the screen is washed with a solvent, in order to remove the ink, masking product and chlorinated product residues, the solvent is left to react and then the screen is rinsed with high-pressure water. After the screen is dried, it is again available for the next use.
Another drawback with the cleaning of the screens is the very high level of pollution that they produce, since the effective solvents such as, for example, halogenated hydrocarbons are harmful and nonbiodegradable and, moreover, the residual mixture of water, solvents and ink that are collected cannot easily be recycled. In addition, these solvents are inflammable and handling them, especially in large quantities, is dangerous.
The problem of the complexity and the duration of the cleaning process has already been studied. Thus, according to document FR-A-2 354 377, a composition for cleaning a screen is known which allows both the ink and the masking product to be removed, this composition consisting of a mixture of an organic solvent, especially gamma-butyrolactone, N-methyl-2-pyrrolidone or dimethylformamide (DMF), an oxidizing agent such as sodium periodate, and optionally a surfactant.
However, the treatment of the screen by this composition is not completely satisfactory as a prior step of treating the screen with white spirit proves to be necessary in order to remove the surplus ink. Furthermore, this solution does not solve the problem of pollution by the residual organic solvents, and it may involve particularly toxic solvents such as DMF.
To limit the pollution caused by the cleaning compositions and their consequent effects on the environment, document WO-A-90/08603 provides a composition in which the organic solvents conventionally employed are replaced with a mixture of dibasic esters, such as dimethyl esters chosen from dimethyl succinate, adipate and glutarate, which is used in the presence of a surfactant. This ester mixture, which acts on the printing ink, is not toxic and is biodegradable. The preferred surfactant is nonionic, such as an ethoxylated aliphatic alcohol or an ethoxylated alkylphenol. One advantageous composition comprises 90-100% by weight of the aforementioned ester mixture and 10-0% by weight of an ethoxylated aliphatic alcohol.
However, this composition does not dissolve the masking product and its action must be supplemented in a conventional manner with a degritting step and then a step of treatment with a chlorinated product in order to remove the ghost image.
For want of a really effective solution, the screen cleaning processes have remained the same for many years, that is to say for at least thirty years, and have been widely employed in screen printing, despite their numerous drawbacks.
According to the invention, a cleaning composition has been developed which makes it possible to solve all the disadvantages encountered with the conventional cleaning methods, including the partial solutions of the prior art.
The invention provides a composition and a process for cleaning a screen-printing screen which are effective and result in a single step, in the removal of both the ink and the masking product. This composition is highly advantageous as it has a specific activity with respect to the products to be removed from the screen, but has no corrosive effect liable to damage the fabric or to result in its premature aging.
Furthermore, the cleaning according to the invention leaves no or virtually no ghost image behind so that the screen, after drying, can be reused immediately.
The benefit of the cleaning power of the composition is twofold. Firstly, and as indicated above, this power is exerted against all the products and residues to be removed from the mesh cells of the fabric, but, in addition, it is almost immediate, and therefore the period of decommissioning of the screen for the sake of cleaning is considerably reduced.
Despite its effectiveness, this composition is not polluting. It is highly advantageous as it is based on nontoxic products presenting no hazard either to the user or to the environment.
Thus, a first subject of the invention is a cleaning composition comprising:
an aqueous solution comprising from 0.05 to 5% by weight of sodium metaperiodate and from 0.05 to 0.75% by weight of sulfuric acid, and
a solution containing from 0.5 to 90% by weight of a mixture of dialkyl esters and from 0.5 to 50% by weight of a nonionic surfactant.
The percentages mentioned in the present description and the claims are expressed by weight of constituents with respect to the weight of the solution when a solution as defined above is defined, or with respect to the final weight of the composition when the composition is defined, as will be revealed in the rest of the description.
Thus, the proportions of the various constituents of a composition of the invention are defined so as to obtain a composition tailored to the ink to be removed.
Advantageously, a composition as defined above furthermore comprises 1 to 15% by weight of periodic acid.
One particular composition of the invention comprises:
0.05 to 5% by weight of sodium metaperiodate,
0.05 to 0.75% by weight of sulfuric acid,
1 to 15% of periodic acid,
0.5 to 90% by weight of a mixture of dialkyl esters, and
0.5 to 50% by weight of a nonionic surfactant.
A composition of the invention furthermore satisfies the following characteristics, considered independently or in combination.
The mixture of dialkyl esters comprises or consists of a mixture of dialkyl glutarate, succinate and adipate; preferably, it is a mixture of dimethyl glutarate, succinate and adipate, the proportions of which are advantageously 61-67% in the case of the glutarate, 20-26% in the case of the succinate and 13-19% in the case of the adipate.
The nonionic surfactant is an ethoxylated alkylphenol and preferably ethoxylated nonylphenol.
One particularly appropriate composition comprises:
an aqueous solution containing from 0.1 to 0.5% by weight of sodium metaperiodate and from 0.1 to 0.5% by weight of sulfuric acid,
an aqueous solution containing from 1 to 20% by weight of a mixture of dialkyl esters and from 1 to 22% by weight of a nonionic surfactant.
A further subject of the invention is a process for cleaning a screen-printing screen, in which the screen-printing screen is exposed to a composition like the one which has just been defined.
For this purpose, before the exposure step, the two aqueous solutions of said composition are mixed together.
Preferably, exposure of the screen to the composition is carried out by spraying (or vaporization), which technique will more particularly be employed if the cleaning is carried out in tunnels; it may also be carried out manually by application, for example, by means of a windowpane moistener.
After the exposure time, which may vary from one to five minutes, the screen is rinsed with water, preferably high-pressure water.
The subject of the invention is also the use of a composition as defined above for cleaning a screen used in screen printing.
Implementation of the process of the invention no longer requires voluminous installations, a single compartment being sufficient to expose the screen to the cleaning composition, in order to rinse it or even to dry it.
Thus, yet another subject of the invention is a machine for cleaning screens (2) used in screen printing, comprising at least a first compartment (4) in which there is a first spray bar (26) having nozzles intended to spray a cleaning composition onto the screen (2) and a second spray bar (32) having nozzles intended to spray high-pressure water, the said compartment (4) being associated with a tank (20) for recovering the cleaning product, said machine furthermore including a cleaning composition reservoir (14) separate from the recovery tank (20).
In this way, the product sprayed onto the screen to be cleaned is a clean product, which contains no diluted ink. This results in more effective cleaning and it is no longer necessary to provide two compartments for cleaning the screen. This structure also makes it possible to use the same tank to recover the degritting product.
The tank for recovering the cleaning composition is preferably fitted with a filtration system so as to comply with the legislation in force governing discharge into the drains. The filtration system will be designed to discharge only product having a COD of less than 2000.
In an advantageous embodiment, the first and second spray bars are placed beside each other so as to form only one unit.
Preferably, the spray bars are arranged horizontally. In this case, it will be preferable to choose to have moveable spray bars and a stationary screen during spraying. The spray bars then move advantageously upward and then downward in order to obtain good cleaning.
To complete the cleaning, the machine advantageously comprises motor-driven rotary brushes, the axes of which are approximately perpendicular to the plane of the screen.
In one embodiment, the spray bars can move with respect to a screen to be cleaned, which remains stationary while the cleaning composition and the water are being sprayed, as already mentioned above. However, it would be possible to have a moveable screen and stationary spray bars. In this case, the spray bars would preferably be vertical.
To allow better cleaning, the machine advantageously comprises a second double spray bar placed in such a way that the screen to be cleaned is located between the two double spray bars and can be sprayed on both its sides.
Advantageously, the machine comprises two metering devices, one taking off a predetermined quantity of a solution of sodium metaperiodate and sulfuric acid, the other taking off a predetermined quantity of a solution of a mixture of dialkyl esters and a nonionic surfactant, said metering devices being fitted with a water feed. Within one of the metering devices an aqueous solution of sodium metaperiodate and sulfuric acid will be obtained, and within the other metering device an aqueous solution of a mixture of dialkyl esters and a nonionic surfactant will be obtained. The products taken off are stored in reservoirs placed inside or outside the machine. The metering devices are hydraulic or electric.
In order also to allow any ghost images to be removed, the machine advantageously comprises a second compartment provided with spray means which can move with respect to the screen. These moveable spray means comprise, for example, a double spray bar, one spray bar being intended to spray a product for removing ghost images and the other spray bar for spraying high-pressure water.
In order to allow the product for removing the ghost images to be rapidly dried, the second compartment is advantageously provided with a forced-air system for driving hot air into the second compartment. To further improve the drying, the second compartment may be provided with a bar, which can move with respect to the screen, allowing hot air to be blown onto the screen. Even better drying is achieved when the first compartment includes a bar which can move with respect to the screen, this bar being a vacuum bar intended to initiate drying of the screen.
To avoid the appearance of streaks on the screen after it has passed through the machine according to the invention, means are advantageously provided for making the screen oscillate. In this way, during the operations relating to rinsing and drying of the screen, a better distribution of the stream over the screen is obtained, thereby preventing the appearance of streaks corresponding to the spray boundaries between two adjacent nozzles.
The invention also relates to a kit for cleaning and degritting a screen used in screen printing, this kit comprising a first chamber containing a solution, optionally an aqueous solution, of an ethoxylated alkylphenol and of dialkyl esters and a second chamber containing an aqueous solution of sodium metaperiodate and sulfuric acid.